The stiffened panels comprise a skin, with thickness about 1/1000th of the smallest dimension of the panel, which panel has an area of about a square meter. The skin is stiffened by stringers with a cross section comprising a bottom flange adapted to be supported on a side of the skin and flanges with cross-sections extending along a plane perpendicular to the bottom flange. For example, the cross-section of such a stiffener is shaped in a “T”, “I”, “L”, “Z” or omega (Ω) shape and assembled to the skin by their bottom flange, so that said sections extend along a direction referred to as the longitudinal direction that is substantially parallel to the largest dimension of the skin. In different exemplary embodiments, said skin is alternatively generally flat in shape, shaped in a developable shape or a non-developable shape called a double curvature shape. Longitudinal stiffeners, commonly referred to as ‘stringers’, extend longitudinally along a direction that is substantially the direction of the lighter curvature of the skin. Finally, the skin may comprise thickness variations that particularly lead to steps on the side of the skin on which the bottom flanges of the stiffeners are supported. While, in the case of a stiffened panel made of metal, the stringers are most often assembled to the skin by riveting, in the case of a panel made of a composite material with fiber reinforcement, the longitudinal stiffeners are preferably assembled to the skin without using mechanical fasteners, particularly by gluing or welding. For example, the document EP-A-2 268 471 describes an assembly method by co-curing a stiffener and a skin to make a stiffened panel, wherein the stiffener and the skin are made of a composite material with fiber reinforcement in a thermosetting matrix. Co-curing of the matrix of the stiffener and the matrix of the skin makes it possible, during the co-curing operation, to make a weld of the stiffener on the skin, where the bond is made by the material making up the thermosetting matrix. This method requires complex tooling, called integral tooling, which can retain the geometric integrity of both the skin and the stiffener during the co-curing operation; the tooling particularly comprises cores placed in the volumes located between the skin and the inside of the flanges of the Omega-shaped stiffener. The tooling, the area of which is at least equal to the area of the panel, is extremely costly to make and is specific to a given panel type; but one aircraft can comprise several hundreds of types of panel.
The document WO 2010/094808 describes a method and device suitable for the localized bonding of a stiffener on a thermosetting composite skin, particularly with a view to repairs. Said device applies pressure on the whole section of the stiffener, so that when said stiffener comprises a hollow section, it must be supported by inserting a core in said section.
Composite materials with a thermoplastic matrix, particularly when the matrix is made of high-performance polymer such as polyetheretherketone or PEEK, have mechanical properties such as impact resistance and fire behavior that make them particularly useful candidates for aeronautics applications, for use as structural materials. However, in order to assemble stiffeners on the skin of such a panel with a thermoplastic matrix, the matrix must be raised to a temperature above its melting temperature. Firstly, that temperature is high, about 400° C. for a PEEK matrix, making the designing of the tool complex, particularly vis-à-vis the control of thermal expansion. Secondly, the need to make the matrix melt and the eventual bulking of the melted polymer make it necessary for the tool to be both sealed and for it to allow the control of volumes, while retaining the placing of reinforcements during the various stages of the assembly operation between the condensed and liquid states of the matrix. These technical problems are obstacles to the use of composite materials with thermoplastic matrices for making large stiffened panels.